Various kinds of motor vehicle hill holding mechanisms have been devised in the past for preventing roll back particularly during shifting while ascending a hill below some predetermined speed of which illustrative examples can be respectively found in U.S. Pat. Nos. 4,650,046 and 4,867,291, the disclosures of which are included herein by reference.
In U.S. Pat. No. 4,650,046 a hill holding device is provided that combines a one-way clutch with a singular pressurized fluid chamber actuated piston that operates to compress a plurality of friction brake plates to brake a motor vehicle transmission output shaft in response to a control signal derived from throttle position, vehicle speed, engine speed and gear selection.
In U.S. Pat. No. 4,867,291 a pressurized fluid actuated one-way clutch is utilized by itself to brake a vehicle transmission countershaft as a function of gear selection.
Hill holding systems commonly employ friction plates that are operative to release torque present in the driveline so that the transmission can be shifted such as when a one-way clutch in the drive line traps torque when the wheels are prevented from rolling when the vehicle pushes against a curb or other stationary obstacle.
Hydraulically actuated pistons used for compressing friction plates together in inertia brake and in hill holder systems have heretofore used a singular pressurized fluid chamber and have thus been limited on the amount of force created which is of importance in view of todays increasing vehicle loads. The present invention provides at least one additional secondary pressurized fluid chamber that enables force to be increased by the amount of cross-sectional area of the secondary chamber exposed to the piston which in turn eliminates the need to raise fluid pressure for singular fluid chamber systems beyond practical limits. The use of at least two pressurized fluid chambers allows the mechanism of the invention to act both as an inertia brake of relatively low torque to slow the speed of the transmission gears connected to the input shaft to synchronize for a normal upshift and to also act as a hill holding brake requiring relatively higher torque and operative to keep the vehicle from rolling backward down a hill when the master clutch is disengaged and the transmission is in a starting gear.